The present invention concerns laser technology and pertains particularly to lenses that launch high bandwidth modes into a fiber optic cable while reducing or eliminating feedback to a laser. A vertical cavity surface emitting laser (VCSEL) emits light in a beam vertically from its surface. Light emitted from an VCSEL is typically focused by a hyperbolic transfer lens (HTL) into an optical fiber and used for transmission of data. Transmission technology such as Gigabit Ethernet technology utilizes VCSELs and multimode fiber optic cabling.
In order to achieve maximum link length, it is desirable that the different fiber modes excited by the laser propagate through a fiber optic cable at the same speed. This allows the light to simultaneously arrive at a destination. However, there are many installed fiber optic cables with various index anomalies that can cause certain bandwidth modes to propagate at speeds different than desired.
In accordance with the preferred embodiment of the present invention, a light transmission system includes a laser, an optical fiber and a transfer lens. The transfer lens transfers light emitted by the laser into the optical fiber. The transfer lens includes a hyperbolic collimating surface for receiving and collimating light originating from the laser. The transfer lens also includes an output lens surface shaped so that light reflected from the transfer lens is not focused at a location at which the light is emitted by the laser. Additionally, in various preferred embodiments light launched into the optical fiber avoids index anomalies on the axis of the optical fiber and index anomalies at the core cladding interface within the optical fiber.
For example, the laser is a vertical cavity surface emitting laser. The output lens surface can be, for example, a toroidal lens surface, a multiple zone lens surface, a spiral Fresnel lens surface or a spherical lens surface.
In these light transmission systems, optical feedback to the laser is reduced. The various embodiments of the present invention can insure a low coupled power reflected from the end of the optical fiber to the laser sufficient to insure satisfactory operation. The low two pass transmissivity back to the laser of the output lens surface can decrease the feedback power by as much as 25 dB. If too much power is coupled back into the laser from the reflection from the end of the optical fiber, instabilities occur in the laser and the output power oscillates up and down, causing extra and damaging amounts of jitter as the received signal pulses.